Object layout

World w = new World(400, 400);

World

public class World {

  private int width;
  private int height;

  public World(int width, int height) {
    this.width = width;
    this.height = height;
  }

  // A bunch of other stuff
}

Turtle yertle = new Turtle(50, 75, w);

Turtle

public class Turtle {

  private int x;
  private int y;
  private World world;

  public Turtle(int x, int y, World world) {
    this.x = x;
    this.y = y;
    this.world = world;
  }

  // A bunch of methods
}

Turtle myrtle = new Turtle(200, 300, w);

Turtle nobody = null;

World w = new World(400, 400);

Turtle yertle = new Turtle(50, 75, w);

Turtle myrtle = new Turtle(200, 300, w);

Turtle nobody = null;